The present invention relates to a novel material and method for the formation of a thermal-spray coating layer of a rare earth fluoride or, more particularly, to an efficient method for the formation of a highly corrosion-resistant thermal-spray coating layer of a rare earth fluoride on the surface of a substrate.
The method of so-called thermal spray coating utilizing a gas flame or plasma flame is a well established technology for the formation of a coating layer having high heat resistance, abrasion resistance and corrosion resistance on the surface of a variety of substrates such as articles made from metals, concrete, ceramics and the like, in which a powder to form the coating layer is ejected or sprayed as being carried by a flame at the substrate surface so that the particles are melted in the flame and deposited on the substrate surface where the melt is solidified to form a coating layer solidified by subsequent cooling.
While oxide powders of aluminum, chromium and the like are employed as the conventional material of the thermal-spray coating layers, these oxide materials are not always quite satisfactory to withstand the plasma atmosphere of a halogen-containing etching gas at an elevated temperature frequently encountered in the manufacturing process of semiconductor devices in respect of the corrosion resistance.
Namely, it is very conventional that the manufacturing process of semiconductor devices involves a step of plasma etching using a halogen-containing gas. Examples of the halogen-containing or fluorine- and/or chlorine-containing gases used for plasma etching include SF6, CF4, CHF3, ClF3, HF, Cl2, BCl3 and HCl either singly or as a mixture of two kinds or more.
It is known that a thermal-spray coating layer having particularly high resistance against corrosion in a plasma atmosphere of these halogen-containing gases can be obtained by using particles of aluminum fluoride as the material of coating among the above mentioned metal oxide powders. While the aluminum fluoride powder used in the prior art for the thermal-spray coating is prepared by pulverizing or grinding coarse particles of aluminum fluoride and used as such, such a powder is generally not satisfactory for the purpose due to the poor flowability sometimes to cause clogging of the feed tubes of the powder and the spray nozzles greatly decreasing the smoothness of the thermal-spray coating procedure or greatly decreasing the quality of the thermal-spray coating layer.
In view of the above described problems and disadvantages in the material and method of thermal-spray coating using particles of aluminum fluoride as the thermal spray powder, the present invention has an object to provide an improvement in the thermal-spray coating method with particles of a rare earth fluoride as the thermal spray powder by which the thermal-spray coating works can be conducted very smoothly and efficiently with high stability to give high productivity of the process and high quality of the thermal-spray coating layer.
Thus, the present invention provides a material and method for the formation of a highly corrosion-resistant coating layer of a rare earth fluoride on the surface of a substrate by thermal-spray coating, the method comprising the step of:
spraying particles of a rare earth fluoride at the substrate surface as being carried by a flame or, in particular, plasma flame to deposit a melt of the particles on the substrate surface forming a layer, in which the rare earth fluoride particles have a globular particle configuration having an average diameter in the range from 20 to 200 xcexcm and are prepared by granulation of primary particles of the rare earth fluoride having an average particle diameter not exceeding 10 xcexcm.
As is described above, the most characteristic feature of the inventive method consists in the use of granules of a rare earth fluoride, as a novel material for thermal spray coating, having a specified average particle diameter as prepared by granulation of primary particles of the rare earth fluoride having a specified average particle diameter. The average particle diameter of the rare earth fluoride granules should be in the range from 20 to 200 xcexcm. When the average particle diameter of the rare earth fluoride granules is too small, a substantial portion of the rare earth fluoride forming the granules may eventually be evaporated in the flame during the thermal-spray coating procedure resulting in a decrease in the utilizability of the thermal-spray powder while, when the average diameter of the granules is too large, large granules cannot be melted completely to the core in the flame resulting in a decrease in the quality of the thermal-spray coating layer of the rare earth fluoride.
The inventors have further continued investigation on the physical properties required for the rare earth fluoride granules and arrived at an unexpected discovery that the granules should have a powder compression factor S not exceeding 30%. The term of xe2x80x9cpowder compression factorxe2x80x9d S here implied is defined by the equation
Powder compression factor S(%)=(xcfx84Txe2x88x92xcfx84A)/xcfx84Txc3x97100, 
in which xcfx84 A is the bulk density of the granules and xcfx84T is the tapped density of the granules.
Although the thermal-spray powder used according to the invention consists of granules obtained by granulation of primary particles of a rare earth fluoride, it is desirable that the granules have a relatively low porosity or small void spaces. This requirement is important to ensure good resistance of the granules against cracking in handling and to decrease emission of a gas occluded in the void spaces of the granules by the thermal spraying to cause melting of the granules.
In order to ensure little emission of gases from the granules melted in the flame, it is desirable that the content of water or hydroxyl groups in the rare earth fluoride particles should be as low as possible since water or hydroxyl groups may be an emission source of gases and may react with the rare earth fluoride to form corrosive hydrogen fluoride gas heavily affecting the quality of the thermal-spray coating layers of the rare earth fluoride. In this regard, the content of water and hydroxyl groups in the rare earth fluoride granules should not exceed 1% by weight.
The primary particles of a rare earth fluoride, from which the granules are prepared by using a suitable granulator machine such as a spray-drying granulator, can be prepared by mechanical grinding and particle size classification and should have an average particle diameter not exceeding 10 xcexcm or, preferably, not exceeding 5 xcexcm or, more preferably, not exceeding 3 xcexcm. This requirement is important in order to obtain rare earth fluoride granules of a globular particle configuration having good flowability and suitable for the thermal-spray coating according to the present invention.
The rare earth elements forming the rare earth fluoride include yttrium and the elements having an atomic number in the range from 57 to 71, of which yttrium, cerium and ytterbium are preferable, though not particularly limitative thereto. These rare earth elements can be used either singly or as a combination of two kinds or more according to need. When two kinds or more of the rare earth elements are combined in the fluoride granules, it is optional that the granules are prepared by granulation of a blend of primary particles of the two kinds or more of different rare earth fluorides or the primary particles per se are prepared to include two kinds or more of the rare earth elements.
The procedure for the preparation of the rare earth fluoride granules from the primary particles is rather conventional. Namely, the primary particles of the rare earth fluoride are dispersed in water, an alcohol of 1 to 4 carbon atoms in a molecule, toluene, hexane and the like or a mixture thereof as a dispersion medium to give a slurry of 10 to 40% solid concentration which is subjected to spray-drying granulation to give granules of the rare earth fluoride. The granules are then subjected to a heat treatment at a temperature not higher than 500xc2x0 C. or, preferably, in the range from 70 to 200xc2x0 C. in vacuum or in an atmosphere of air or an inert gas such as argon, nitrogen and helium with an object to decrease the moisture content therein. The thus obtained granules are further subjected to a calcination treatment for 30 minutes to 5 hours in air at a temperature not exceeding 600xc2x0 C. or, preferably, in the range from 300 to 500xc2x0 C.
It is optional that the aqueous dispersion of the primary particles of a rare earth fluoride is prepared by using water containing a water-soluble organic resin such as carboxymethyl cellulose as a binder of the particles in order to facilitate granulation. The granules obtained in this way naturally contain the binder resin. It is optional that such binder resin-containing granules are subjected to a heat treatment in order to decrease the carbon content therein by burning-off of the binder resin.
Besides the above mentioned carboxymethyl cellulose as the binder resin, polyvinyl alcohol, polyvinyl pyrrolidone and the like can also be used as the binder resin. The amount of the binder resin, when used, is in the range from 0.05 to 10% by weight based on the amount of the primary particles of the rare earth fluoride.
The thermal-spray coating according to the present invention is performed preferably by the plasma thermal-spray coating or reduced-pressure plasma thermal-spray coating in which the plasma gas can be selected from argon and nitrogen as well as gaseous mixtures of nitrogen and hydrogen, argon and hydrogen, argon and helium, argon and nitrogen and the like, though not particularly limitative thereto.
The material forming the substrate, to which the method according to the present invention is applicable typically including members and parts of a semiconductor-processing instrument, is not particularly limitative and can be any of metallic materials such as aluminum, nickel, chromium and zinc as well as alloys of these metals and ceramic materials such as alumina, aluminum nitride, silicon nitride, silicon carbide and fused silica glass. The thermal-spray coating layer has a thickness usually in the range from 50 to 500 xcexcm.
It is optional that the rare earth fluoride powder, with which the thermal-spray coating according to the present invention is undertaken, is admixed with a limited amount of a rare earth oxide powder which desirably has a particle size distribution close to that of the rare earth fluoride granules.